The heart is a hollow, muscular organ that pumpsblood through the vessels and cavities of the body by its repeated, rhythmic contractions. In the human body the heart is situated in the middle of the thorax[?] and weighs about 300-350g in an adult. It consists of four chambers, the two upper atria and the two lower ventricles. Mammals have a doublecirculatory system, meaning that two separate circulatory cycles occur simultaneously. The first cycle, termed pulmonary circulation, consists of deoxygenated blood being pumped out of the right ventricle into the lungs, where it exchanges unwanted carbon dioxide and water for oxygen. The blood then travels back into the left side of the heart, from where it is pumped all round the body in systemic circulation. The body removes oxygen and nutrients from the blood to be used in respiration and the cycle continues.

The atria chambers receive blood from the veins, and the ventricles pump it from the heart into the arteries. The left ventricle is much thicker than the right because it must pump blood around the entire body, this involves exerting a considerable force to overcome the pressure caused by the body. As the right ventricle must just pump blood to the lungs it requires less muscle.

The wall of the heart is very muscular and does not tire. It consists of three distinct layers. The first is the outer epicardium[?] which is composed of a layer of flattened epithelial cells[?] and connective tissue. Beneath this is a much thicker myocardium[?] made up of cardiac muscle. The endocardium[?] is a further layer of flattened epithelial cells and connective tissue.

Cardiac muscle is myogenic. This means that unlike skeletal muscle[?] which requires either conscious or reflex nervous stimuli, cardiac muscle is self-exciting. The rhythmic contractions occur spontaneously, although the frequency can be changed by nervous or hormonal influences such as exercise or the perception of danger. A large blood supply is necessary to power the heart and this is supplied by the left and right coronary arteries, which branch off from the aorta.

Every beat of the heart involves a sequence of events called the cardiac cycle. This consists of three major stages: the atrial systole, the ventricular systole and the complete cardiac diastole. The atrial systole consists of the contraction of the atria and the corresponding influx of blood in to the ventricles. Once the blood has fully left the atria atrioventricular valves, which are situated between the atria and ventricular chambers, close. This prevents any backflow into the atria. It is the sound of the valves closing which produces the familiar beating sounds of the heart.

The ventricular systole consists of the contraction of the ventricles and flow of blood in to the atria. Again, once all the blood has left, the pulmonary and aortic semilunar valves close. Finally complete cardiac diastole involves the relaxation of the atria and ventricles in preparation for new blood to enter the heart.

The heart is self-regulating, the rhythmic sequence of contractions is coordinated by the sinoatrial node and atrioventricular nodes. The sinoatrial node, often known as the pacemaker, is located in the upper wall of the right atrium and is responsible for the wave of electrical stimulation (See action potential) that initiates the atria to contract. Once the wave reaches the atrioventricular node, situated in the wall between ventricular chambers, it is conducted through the bundles of His and causes contraction of the ventricles. The time taken for the wave to reach this node from the sinoatrial nerve creates a delay between contraction of the two chambers and ensures that each contraction is coordinated simultaneously throughout all of the heart.